The use of halftones in printing processes to reproduce photographs, paintings and other continuous tone original copy is well known, and the art has been widely established for many years. A halftone is produced by placing a screen in contact with a negative within a camera, mounting the continuous tone original copy on a copyboard in front of the camera, and then exposing the copy through the screen. The screen is designed to break the continuous tone original copy into a great multiplicity of very small dots, which produces a negative similarly having such dots and which is then used to make a printing plate.
A typical screen will be comprises of two sheets of glass having a large number of parallel opaque lines thereon, the sheets being placed in contact with the lines on one sheet running at 90.degree. to the lines on the other sheet, whereby a great many very small squares are formed through which reflected light can pass to reach a negative. The number of lines on a screen can vary, depending on the nature of the copy, the desires of the operator, the kind of paper to be used in the printing process, and other factors. Typically, screens will have from 45 to 400 lines per inch, although screens with from 60 to 150 lines are more commonly used. Screens are also known with circular and eliptical dots, as opposed to square, and such offer advantages in certain situations.
In making a halftone by the conventional single negative process, the continuous tone original copy is first secured to a copyboard in line with the lens of the camera, and a suitable screen is placed in the camera in contact with a negative. Then, a main exposure is made, wherein the camera lens is opened and the copyboard is lighted. Light is reflected from the original copy, through the camera lens and the screen, and onto the negative. The screen serves to break the reflected light into dots, the lightest copy areas reflecting the most light and casting the largest, most closely spaced dots on the negative, and the darker copy areas reflecting less light and forming smaller, more widely spaced dots.
When the negative is then developed and used to make a printing plate, the dot pattern on the plate is reversed. That is, the darker areas will have the greater dot intensities and thus will apply the most ink to the paper, while the highlight areas will contain the fewest dots and will apply the least ink. An ink print will contain a reproduction of the original copy, appearing as a pattern of minute dots. To the naked eye, the dot patterns on the ink print create the illusion of continuous tone. However, the dot pattern can be seen under heavy magnification.
For some continuous tone original copy, the main exposure alone will produce a satisfactory negative. However, it has been found that in most instances, the halftone resulting from the main exposure provides a less than satisfactory reproduction of the original copy, particularly when the original is a photograph containing a wide range of tones on the gray scale.
The commonly used gray scale measures tones on a value scale of from 0 to 2.00, with 0 representing white and 2.00 representing black. Between 0 and 2.00, the gray scale is normally divided into ten tones of varying intensity, commonly expressed by the density designations of 0.10, 0.20, 0.30, 0.50, 0.70, 1.00, 1.30, 1.60, 1.90 and 2.00. Devices have been developed for measuring the tone intensity of copy, commonly called densitometers, and these can provide a readout of density on the gray scale. The highlight and middletone areas will typically be in the range of from 0 to about 0.90 on the gray tone scale for an average photograph. The darker shadows will fall in the upper end of the scale between 1.60 and 2.00, with 2.00 being black.
The problem in making good halftones from continuous tone original copy containing a wide range of gray tones is as follows. If the main exposure is too long, so as to allow all of the middletone and darker areas to be reflected by dots on the negative, then the whiter highlight areas will become a mass of closely spaced large dots and will wash out when printed. On the other hand, if a short main exposure is made, there will be very few or no dots on the negative in the upper middletone and darker areas, and all definition in these portions of the copy will be lost. Thus, the typical main exposure must be a compromise, unless the original copy has a narrow range of gray tones thereon. The result is that the dot pattern in most instances is not truly satisfactory in either the highlight or the darker areas, while the middletones may be generally accurate. The quality of an ink print is thus also compromised, and it will not be a true reproduction of the original copy.
In an effort to overcome this problem, techniques known as "flashing" and "bumping" have been developed, among others. A flash is a short, supplementary nonimage exposure of the negative, made through the screen, and is intended to compensate for the lack of light reflected from the darker parts of the original copy during the main exposure. A flash is commonly done by first placing a white card over the original copy on the copyboard, and then making a short exposure of the negative with the screen in place. The result is to burn additional dots on all areas of the negative, resulting in dots where none may have previously existed in the darker areas, and the enlargement of dots made earlier during the main exposure.
In addition to using the white card technique, flashing is also sometimes done by shining a light through the camera lens for a short period, with the screen in place, or by shining an overhanging light on the screen and negative sandwich exposed on an open camera back. In the latter instance, the overhanging light is usually fitted with a yellow filter.
As has been noted, flashing provides a supplemental exposure through the screen, and affects all areas of the negative. While producing more intense dots in the middletone and shadow areas, it also produces shadow dots in the very dark areas, and enlarges the dots in the white or highlight areas. The result is to improve overall detail in the resultant print, but at the price of some deterioration in all of the gray tones, a general flattening out of the reproduction, and the creation of a general haze over the print reproduction. By adding shadow dots in the dark portions there is a resultant reduction in ink saturation in such areas, sometimes by as much as 0.40 on the gray scale; thus, blacks on the original copy may appear dark gray on the reproduction. While there is a loss of detail if the shadow dot is missing, there is an accompanying loss of contrast when the shadow dots are burned on the negative. Thus, while flasing can provide an improved print over use of only a main exposure, the resultant gray tones of the reproduction are again a compromise and do not truly reflect the original copy.
In addition to flashing, the further technique of bumping is sometimes used. A bump is a very short exposure of the continuous tone original to the negative, with the screen removed from the copy camera. The effect of a bump is to close up the tiny clear dots in the negative, or to perhaps simply drop them out. This will produce more contrast in that area, but with loss of detail.
The commonly used single negative halftone techniques, then, can provide acceptable printing plates for most instances. But the quality of the reproduction is not truly faithful to the original copy for the reasons noted. The typical reproduction print will contain shadow areas which are more poorly defined than on the original copy, and it will have a general light haze thereover.
A different technique which can produce halftones of higher quality has also been proposed, wherein a plurality of negatives are made. Each negative has a different exposure, to accurately represent one portion of the gray scale as such is contained on the original copy. The negatives are then combined to make the reproduction. While this multiple negative process produces better results than the single negative process just described, it requires multiple exposures and is expensive. Further, considerable skill is required to practice the technique, since there must be precise registration of the different negatives. Thus, the multiple negative halftone process does not fully satisfy the need for an effective halftone technique.
There is need for an improved process and apparatus for making halftones, capable of producing reproductions of original copy that are faithful in tone, well defined and haze-free, and which will not require expensive new equipment or modifications to existing equipment, or great skill to utilize. The present invention is intended to satisfy that need.